The long-term goal of this project is to understand the molecular mechanisms that control gene expression and developmental transitions. While transcription has been extensively studied, the posttranscriptional mechanisms of RNA alternative splicing is much less understood despite of their importance in cellular regulation, human health, and plant growth and development. We have discovered that the Arabidopsis protein AtAcinus is evolutionarily related to but highly divergent from the human Acinus protein, which plays important roles in regulating transcription, RNA alternative splicing, and apoptosis. Our unpublished studies have shown that AtAcinus is modified by O- GlcNAcylation, plays essential role in alternative splicing of a number of genes, many of which encoding key components of signaling and developmental pathways. In particular, our data indicate that AtAcinus play important roles in regulating seed germination and flowering, two major developmental transition in plants. Using a combination of proteomics, genetics, genomic and biochemical approaches in the Arabidopsis model system, we have made tremendous progress in understanding the functions of AtAcinus. Our results support a hypothesis that AtAcinus is controlled by O-GlcNAcylation in response to endogenous and environmental cues, and in turn it regulates key cellular pathways through both transcriptional and posttranscriptional mechanisms. In this proposal, we plan to continue using the combination of proteomic, genomic and genetic approaches to further advance our understanding of Acinus regulatory pathway. We will 1) dissect the molecular functions of AtAcinus, particularly taking advantage of proximity labeling, cross-linking mass spectrometry and biochemical fractionation, CLIP-seq and CLIP-MS technologies to understand how AtAcinus carries out multiple functions (aim 1 and 3); 2) dissect how AtAcinus functions are regulated by post- translational modifications (aim 2). The experiments outlined in this proposal will greatly advance our understanding of the molecular mechanism of RNA alternatively splicing and O-GlcNAcylation and the mechanisms of signal integration at post-transcriptional level. Given the evolutionary conservation of Acinus, this study not only is important for plant biology and agriculture, but also can potentially help us understand fundamental mechanisms of signaling and cellular regulation that are relevant broadly.